In the past, the main welding method in the automobile field has been spot welding. Steel sheets have been joined by spot welding to assemble the chassis and produce parts.
For example, in an automobile chassis, mild steel sheet excellent in shapeability and paintability has been used for outside panel parts with high design property. High strength steel sheet has been used for frame parts for ensuring inside strength and rigidity. Usually, mild steel sheet with a relatively thin sheet thickness and high strength steel sheet with a relatively thick sheet thickness are applied.
Therefore, portions arise where thin mild steel sheet and thick high strength steel sheet are joined by spot welding. At this time, sometimes the nugget (weld metal) is not sufficiently formed at the interface between the thin sheet and thick sheet.
PLT 1 proposes joining a plurality of steel sheets by resistance spot welding during which performing the resistance spot welding in two stages and performing the second stage of welding by a higher pressing force, lower current or same current, and longer conduction time or same conduction compared with the first stage of welding.
However, with the method of PLT 1, the pressing force at the time of the first stage of welding is low, so there is a sheet gap between the steel sheets (gap between steel sheets). Further, if the sheet thickness ratio (=total sheet thickness (mm)/thinnest sheet thickness (mm)) is large, sometimes a nugget is not sufficiently formed at the interface of a thin sheet and thick sheet and the desired joint strength cannot be obtained.
PLT 2 proposes a resistance welding method of high strength steel sheet for resistance welding a plurality of superposed steel sheets including at least one high strength steel sheet. Specifically, it proposes a resistance welding method of high strength steel sheet comprising a first step of running current to form a nugget having a nugget size of 3√t to 5√t (t: smallest sheet thickness of steel sheets (mm)), a second step of then lowering the welding current, and a third step of then running a welding current larger than the welding current of the first step to enlarge the nugget.
In the method of PLT 2, the pressing force is increased in the second step, but if there is a sheet gap and the sheet thickness ratio is high, it is difficult to secure the desired nugget size.
PLT 3 proposes a resistance welding method of steel sheet for resistance welding a plurality of superposed steel sheets including at least one high strength steel sheet. Specifically, it proposes a resistance welding method of high strength steel sheet comprising a first step of running current to form a nugget having a predetermined nugget size, a second step of then lowering the welding current, and a third step of then further running a welding current larger than the welding current of the first step and imparting a pressing force larger than the pressing force of the first step to enlarge the nugget.
In the method of PLT 3, to enlarge the nugget, at the third step, a pressing force larger than the pressing force of the first step is imparted, but in the same way as the method of PLT 2, if there is a sheet gap and the sheet thickness ratio is high, it is difficult to secure the desired nugget size.
PLT 4 proposes a method of spot welding three or more steel sheets having a gap G (2 (mm)) between steel sheets by multistage conduction comprising performing first conduction by upslope conduction then performing second conduction by the required conduction time and current value.
In the method of PLT 4, the current of the second stage is made smaller than the maximum current of the first stage, so the interface between a thin sheet and thick sheet is insufficiently melted, a predetermined shape of nugget is not obtained, and the desired joint strength sometimes cannot be obtained.
In this way, when superposing a plurality of steel sheets with a sheet gap and a high sheet thickness ratio, it is difficult to sufficiently melt the contact interface of a thin steel sheet and a thick steel sheet and sufficiently form the nugget and sometimes the desired joint strength cannot be obtained.